1. Field of the Invention
The present invention relates to high performance data cables employing twisted pairs of insulated conductors as the transmission medium. More particularly, the present invention relates to such cables having improved crosstalk performance by use of techniques to separate the twisted pairs from each other and from the cable jacket.
2. Related Art
High performance data cable using twisted pair transmission media have become extremely popular. Such cable constructions are comparatively easy to handle, install, terminate and use. They also are capable of meeting high performance standards.
One common type of conventional cable for high-speed data communications includes multiple twisted pairs. In each pair, the wires are twisted together in a helical fashion forming a balanced transmission line. When twisted pairs are placed in close proximity, such as in a cable, electrical energy may be transferred from one pair of the cable to another. Such energy transfer between pairs is undesirable and is referred to as crosstalk. Crosstalk causes interference to the information being transmitted through the twisted pair and can reduce the data transmission rate and can cause an increase in the bit error rate. The Telecommunications Industry Association (TIA) and Electronics Industry Association (EIA) have defined standards for crosstalk in a data communications cable including: TIA/EIA-568-A, published Oct. 24, 1995; TIA/EIA 568-A-1 published Sep. 25, 1997; and TIA/EIA 568-A-2, published Aug. 14, 1998. The International Electrotechnical Commission (IEC) has also defined standards for data communications cable crosstalk, including ISO/IEC 11801 that is the international equivalent to TIA/EIA 568-A. One high performance standard for data communications cable is ISO/IEC 11801, Category 5.
Crosstalk is primarily capacitively coupled or inductively coupled energy passing between adjacent twisted pairs within a cable. Among the factors that determine the amount of energy coupled between the wires in adjacent twisted pairs, the center-to-center distance between the wires in the adjacent twisted pairs is very important. The center-to-center distance is defined herein to be the distance between the center of one wire of a twisted pair to the center of another wire in an adjacent twisted pair. The magnitude of both capacitively coupled and inductively coupled crosstalk varies inversely with the center-to-center distance between wires, approximately following an inverse square law. Increasing the distance between twisted pairs will thus reduce the level of crosstalk interference. Another important factor relating to the level of crosstalk is the distance over which the wires run parallel to each other. Twisted pairs that have longer parallel runs will have higher levels of crosstalk occurring between them.
In twisted pairs, the twist lay is the longitudinal distance between twists of the wire. The direction of the twist is known as the twist direction. If adjacent twisted pairs have the same twist lay, then the coupling is longitudinally additive. If twisted pairs have opposite twist directions, then they interlace, and their center lines will lie more closely together than they would within a cable in which all pairs have the same twist direction. Thus due to the reduced center to center distance twisted pairs having opposite twist directions will have reduced crosstalk performance. In other words, the crosstalk tends to be higher between pairs having substantially the same twist lay and opposite twist direction.
Therefore, adjacent twisted pairs within a cable are given unique twist lays and the same twist directions. The use of unique twist lays serves to decrease the level of crosstalk between adjacent twisted pairs.
Sometimes, it would be advantageous to also use twisted pairs with opposing twist directions. However, as outlined above, the interlacing between twisted pairs having essentially the same or similar twist lay lengths will increase, thus reducing the crosstalk performance.
Even if each adjacent twisted pairs in cable has a unique twist lay and/or twist direction, other problems may occur. In particular, during use mechanical stress may interlink adjacent twisted pairs. Interlinking occurs when two adjacent twisted pairs are pressed together filling any interstitial spaces between the wires comprising the twisted pairs. Interlinking will cause a decrease in the center-to-center distance between the wires in adjacent twisted pairs and can cause a periodic coupling of two or more twisted pairs. This can lead to an increase in crosstalk among the wires in adjacent twisted pairs within the cable.
One popular cable type meeting the above specifications is foil shielded twisted pair (FTP) cable. FTP cable is popular for local area network (LAN) applications because it has good noise immunity and a low level of radiated emissions.
Another popular cable type meeting the above specifications is unshielded twisted pair (UTP) cable. Because it does not include shield conductors, UTP cable is preferred by installers and plant managers as it is easily installed and terminated. The requirements for modem state of the art transmission systems require both FTP and UTP cables to meet very stringent requirements. Thus, FTP and UTP cables produced today have a very high degree of balance and impedance regularity. In order to achieve this balance and regularity, the manufacturing process of FTP and UTP cables may include twisters that apply a back torsion to each wire prior to the twisting operation. Therefore, FTP and UTP cables have very high impedance regularities due to the randomization of eventual eccentricities in a twisted wire pair during manufacturing.
In order to obtain yet better crosstalk performance in FTP and UTP cables, for example to meet future performance standards, such as proposed category 6 standards, some have introduced a star or cross-shaped interior support for the data cable, such as disclosed by Gaeris et al. in U.S. Pat. No. 5,789,711, issued Aug. 4, 1998.
In conventional cables, the loss factor or loss tangent of the jacketing material has a substantial impact upon the attenuation figure of data grade cables. Attenuation increases with proximity of the transmission media to the jacket. For this reason, data cables not having an interior support such as disclosed by Gaeris et al. generally have loose fitting jackets. The looseness of the jacket reduces the attenuation figure of the cable, but introduces other disadvantages. For example, the loose fitting jacket permits the geometric relationship between the individual twisted pairs to vary, thus varying impedance and crosstalk performance.
In FTP cable, the effect of the loss tangent of the jacketing material is substantially mitigated by the shield. The shielding characteristics of the foil surrounding the twisted pairs determine the effect upon different frequencies. This shielding characteristic is best described by the transfer impedance. However, measurement of the transfer impedance is difficult, especially at higher frequencies.
The performance of shielded cable can be substantially improved by individually shielding the twisted pairs. However, such cables commonly designated as STP (Individually Shielded Twisted Pairs) wires are impractical, as they require a substantial amount of time and specialized equipment or tools for termination. Additionally, the cables themselves are relatively large in diameter due to the added bulk of the shield, which is a severe disadvantage, primarily with respect to causing poor flammability performance, but also with respect to space requirements in ducts and on cross connects.
Conventional interior supports have the basic cross form with parallel sides, such as shown in FIG. 1 or a simple star shape, such as shown in FIG. 2. These shapes have a number of disadvantages, discussed below.
The conventional cable configuration of FIG. 1 includes an interior support 101, a plurality of twisted pairs 102 of insulated conductors 103. Interior support 101 has arms 104 with straight, parallel sides. The entire assembly is surrounded by a jacket (not shown) and possibly by a shield (optional, not shown).
During the stranding operation, in which twisted pairs 102 and the interior support 101 are brought together and twisted into a cable form, the interior support is oriented to the twisted pairs 102 so they can be laid up into the required positions. Then the interior support 101 and twisted pairs 102 are stranded, together. The helical deformation of the interior support 101 stretches the outer, peripheral parts of the support more than the inner parts of the support. This is indicated in FIG. 1 by the dashed lines 105. As the outer peripheral parts of the interior support are stretched and thus thinned, the space in which each individual twisted pair 102 can move is increased. The twisted pairs 102 can move either tangentially to the circumference of the cable or radially, away from the center of the cable. This movement is undesirable, as it causes crosstalk and attenuation variation. Due to the latter, impedance also varies, exhibiting some roughness. Crosstalk is mainly influenced by tangential displacements of the twisted pairs, assuming each pair has a unique lay length to reduce crosstalk. The tangential displacement varies the spacing between pairs. Radial displacement predominantly affects attenuation. Variation in radial displacement cause attenuation variation, also called attenuation roughness, as the distance from the center of each twisted pair to the jacket varies. Both of these variations also incidentally have an impact upon impedance roughness.
The cable shown in FIG. 2 is that disclosed by Gaeris et al. This configuration has an interior support 201 having a plurality of arms 202 with angled sides, giving the interior support an overall star shape. The arms 202 of interior support 201 separate a plurality of twisted pairs 203 of insulated conductors 204. The assembly is shielded by a foil shield 205, and protected by a jacket 206.
The present invention provides an improved high performance data cable including a generally cross-shaped core.
According to one aspect of the invention, a high performance data cable includes a plurality of twisted pairs of insulated conductors; a generally cross-shaped core having arms with flanged ends extending sufficiently far around each twisted pair of insulated conductors to retain each twisted pair of insulated conductors; and a jacket generally surrounding the plurality of twisted pairs of insulated conductors and the core; whereby the plurality of twisted pairs of insulated conductors are held in stable positions apart from each other and from the jacket. In some embodiments of the cable, adjacent arms define a substantially circular void in which a twisted pair of insulated conductors is retained. In other embodiments of the invention, adjacent arms define a substantially polygonal void in which a twisted pair of insulated conductors is retained.